1). Field of the Invention
The present invention relates to a superconductor.
2). Discussion of Related Art
The discovery of high critical temperature (Tc) superconducting ceramics (HTS ceramics) has inspired an enormous interest in their application. Conventional niobium alloy superconductors such as NbTi must be cooled to below 10K to achieve useful superconductivity. HTS superconductors, on the other hand, can have Tcs over 100K. Due to the great expense of cryogenic refrigeration, the HTS ceramics could find much wider application in industrial and laboratory devices. Of particular interest are materials which have Tc above 77K, because this is the temperature of liquid nitrogen, a common and relatively inexpensive refrigerant.
HTS ceramics have not been used in many potential applications because they suffer from a number of shortcomings. The most severe problems with the HTS ceramics are as follows:                1) HTS ceramics are brittle. They are not flexible and thus cannot readily be made into wires or other useful shapes. Cracks and boundaries between adjacent crystals severely limit supercurrent flow.        2) HTS ceramics are highly anisotropic. Supercurrents preferentially flow in certain directions with respect to the crystal lattice, reducing the maximum current density in randomly oriented multicrystalline pieces.        3) HTS ceramics are strong oxidizing agents. Most metals, such as copper, lead, tin, aluminum, indium, and niobium, are oxidized by contact with the ceramic superconductors. Insulating oxide layers impede supercurrent flow. Only noble metals such as gold, silver palladium and their alloys are not oxidized by the HTS ceramics.        
A less severe undesirable feature of the HTS ceramics is that they can lose their superconducting properties under certain circumstances. The superconducting structure inside the HTS ceramics has an abundance of oxygen atoms which are necessary for superconductivity. Heating, grinding, etching, or prolonged exposure to ambient atmosphere or vacuum may liberate the oxygen and destroy superconductivity. Both the oxygen content and the superconductivity can be restored by annealing the HTS ceramic in an atmosphere of oxygen.
It would be an advance in the art of applied superconductivity to provide a superconducting wire employing HTS ceramics that is ductile, has a high Tc, and has a high critical current density (Jc). Such a wire must overcome the problems with the HTS ceramics. Prior art HTS ceramic wires made of a combination of HTS ceramic particles in a silver matrix generally have poor superconducting properties such as low Jc. Also, bending the prior art wires tends to greatly reduce the Jc. This is highly undesirable.
There are other superconductor materials which have some of the same disadvantages as HTS cuprate materials. For example, the A15 family of superconductors such as Nb3Sn are also brittle materials (although they are not anisotropic and relatively nonreactive). Their poor mechanical properties have precluded their use in many applications requiring high current conductors with robust current carrying properties under mechanical stress, such as in a superconducting motor or generator. This is unfortunate because they generally have good superconducting properties such as high critical current densities in high magnetic fields. Also, with a Tc of approximately 18K, Nb3Sn-based superconducting devices are restricted to operation at temperatures less than 10K, which is not economical for many superconducting applications.
Recently, the discovery of superconductivity near 40K in MgB2 has inspired an enormous interest in their application for use in superconducting devices operating between 10K and 30K. Device operation at temperatures greater than 20K precludes the use of the technical superconductors NbTi and Nb3Sn. Similar to HTS ceramics and the A15 superconductors, the poor mechanical properties of MgB2 makes it difficult to fabricate ductile, robust wire or tape with high critical currents. Nevertheless, with a significantly lower materials cost relative to HTS ceramics, the development of high performance MgB2 conductors may enable the economic adoption of superconducting devices such as MRI magnets, transformers, motors, and generators in this intermediate temperature range.
Other examples of brittle, nonductile superconductors include materials possessing the NaCl crystal structure (the AB family), Laves phase ceramics, Chevrel phase ceramics, and metallic borides. These materials may have superior superconducting properties, but are unusable in many applications (e.g., conducting wires) because they are brittle. It would be an advance in the art of applied superconductivity to provide flexible wires made from brittle superconductor materials.
U.S. Pat. No. 5,091,362 to Ferrando discloses a method for forming a silver coating on HTS ceramic particles. U.S. Pat. No. 4,971,944 to Charles et al. teaches a method for electroless deposition of gold onto HTS ceramic particles.
U.S. Pat. No. 5,041,416 to Wilson describes a superconducting composite material. Powders of HTS ceramic and normal metal are mixed and the mixture is subjected to heat and high pressure. The composite materials of Wilson have a relatively low Jc due to reactivity between the HTS ceramics and the metal matrix. The wires also have a low Jc if silver is used as the normal metal.
U.S. Pat. No. 5,202,307 to Hayashi describes a superconducting composite material having HTS ceramic particles in a metal matrix. The composite materials of Hayashi have a relatively low Jc due to reactivity between the HTS ceramic particles and the metal matrix and/or due to poor superconducting properties of the metal matrix materials.
U.S. Pat. No. 5,194,420 to Akihama describes a composite cuprate superconductor/metal superconducting material consisting of HTS ceramic particles dispersed in a matrix of silver. The composite materials of Akihama will also have a relatively low critical current density due to the choice of silver as the metal matrix material.
U.S. Pat. No. 5,081,072 to Hosokawa et al. describes a method preparing a HTS superconducting ceramic powder and forming the powder into a superconducting material. A low Jc is also a problem with the materials of Hosokawa.
U.S. Pat. No. 5,547,924 to Ito et al. describes a superconducting ceramic composite material having HTS ceramic particles in a noble metal matrix. The composite materials have relatively poor superconducting properties due to the poor superconducting properties of the metal matrix materials used.
U.S. Pat. No. 5,132,278 to Stevens et al. describes a cuprate superconductor wire having continuous filaments of HTS ceramic surrounded by metal matrix. A noble metal chemically protects the HTS ceramic. The wires of Stevens et al. are characterized in that they do not conduct current between wires, and do not rely on the superconducting proximity effect.
There exists a need for a superconducting composite material that is ductile and has a high Jc that is not reduced by bending. Also, there exists a need for ductile superconducting composite materials made from brittle superconductors that have a high current carrying properties at high temperatures in modest magnetic fields.